This invention relates generally to nuclear reactors and more particularly to a system for preventing or reducing the leakage out of a contained vessel of fission gases in a gas-cooled nuclear reactor or a liquid cooled nuclear reactor having a cover gas above free surfaces of the reactor coolant.
A nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core situated in a pressure vessel. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. Such nuclear reactors typically comprise one or more primary flow and heat transfer systems and a corresponding number of secondary flow and heat transfer systems to which the conventional steam turbines and electrical generators are coupled. A typical energy conversion process for a commercial nuclear reactor, therefore, involves transfer of heat from a nuclear core to the primary coolant flow system, then to a secondary coolant flow system, and finally into steam from which electricity is generated.
In a liquid cooled nuclear reactor, such as a liquid metal cooled breeder reactor, a reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary system comprises a nuclear core within a reactor vessel, a heat exchanger, a circulating pump, and piping interconnecting the aforementioned apparatus. In nuclear reactors having more than one primary system, the nuclear core and reactor pressure vessel are common to each of the primary systems. The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and trough the reactor core. The heated reactor coolant then exits from the reactor vessel and flows to the heat exchangers which transfer the heat to secondary flow systems associated therewith. The cooled reactor coolant exits from the heat exchanger, then flows to a pump which again circulates the coolant into the pressure vessel, repeating the described flow cycle.
Above the free surfaces of the reactor coolant in the pressure vessel, it is general practice to provide a blanket of inert gas. This gas blanket, normally termed the cover gas, prevents undesirable chemical reactions of liquid metal coolant with constituents of the atmosphere, i.e., oxygen and moisture. In addition, certain components, such as the control rod drive mechanisms, are designed to function in an inert gas atmosphere. To maintain a safer and more efficient operation, this cover gas is maintained at a low positive differential pressure above atmospheric pressure. The circulating pumps of liquid metal-cooled reactors also utilize a cover gas. Here, the cover gas prevents contact of the pump motor and pump seals with the liquid metal coolant. In nuclear reactors equipped with coolant reservoir tanks, a cover gas is generally used above the level of coolant in these tanks.
This cover gas should be maintained in a gas-tight containment area. This is generally accomplished by means of welded or bellows-sealed joints. Certain locations, however, cannot be sealed solely by such means. These locations, such as the joint formed by the pressure vessel and the vessel closure head, must utilize a sealing mechanism to form a gas-tight containment area. The maintenance of such a gas-tight containment area is necessitated by the possibility of a defect in a fuel assembly.
If a defect, such as a crack, occurred in a fuel assembly, radioactive isotopes such as xenon and krypton may be released into the reactor coolant. As these radioactive isotopes have low solubility in sodium, they would flow upward through the coolant and emerge into the cover gas. Without a gas-tight containment area, these radioactive isotopes, or fission gases, could escape into the surrounding environment.
The prior art attempted to solve this problem by utilizing a conventional seal in the joints. This method is not entirely satisfactory. Experimental results have indicated that the cover gas continues to leak through such conventional seals.
Another method employed in the prior art was the use of a buffer gas system. This buffer gas system utilizes a pressurized gas flowing in a channel between such joints. Because this gas is at a higher pressure than the cover gas, any leakage which might occur is forced back into the cover gas containment area by the pressurized gas. A major disadvantage of this system is that many small gas lines are required to supply the pressurized gas flowing in the channel. This buffer gas system adds unwanted complexity to an already crowded area.
A third means of providing a gas-tight containment area for the cover gas is the use of a bellows seal. This bellows seal works satisfactorily in that it prevents all gases from leaking through the joints. The bellows seal has disadvantages, however, in that bellows seals are difficult and expensive to fabricate when the size of the seal required becomes large (approximately 12 inches or more).
This invention provides a system to remove any short-lived radioactive isotopes in the cover gas which could possibly leak to the external environment through the various joints. This system functions for large sealing locations, and does not add substantial additional complexity to the nuclear reactor.